The group was made up of high-ranking retired members of the air force with some of the messages leaked in December to the Infolibre news website, sparking public outrage.

The messages focused on the government of Prime Minister Pedro Sanchez, whose Socialists rule alongside the hard-left Podemos in Spain's first coalition government since the death of dictator Francisco Franco in 1975.

"I don't want these scoundrels to lose the elections. No. I want them and all of their offspring to die," wrote one.

"For them to die, they must be shot and 26 million bullets are needed," wrote another, referring to the number of people who cast their ballots in favour.

Prosecutors opened their investigation in mid-December after finding the statements were "totally contrary to the constitutional order with veiled references to a military coup".

But they dropped the probe after concluding the content of the chat did not constitute a hate crime by virtue of the fact it was a private communication.

"Its members 'freely' expressed their opinions to the others 'being confident they were among friends' without the desire to share the views elsewhere," the Madrid prosecutors office said.

The remarks constituted "harsh" criticism that fell "within the framework of freedom of expression and opinion," it said.

The decision is likely to inflame protests that erupted in mid-February over the jailing of a Spanish rapper for tweets found to be glorifying terrorism, a case that has raised concerns over freedom of speech in Spain.

According to Infolibre, some of the chat group also signed a letter by more than 70 former officers blaming the Sanchez government for the "breakdown of national unity" that was sent to Spain's King Felipe VI in November.

Such remarks echo criticism voiced by Spain's rightwing and far-right opposition that has denounced the government for courting separatist parties in order to push legislation through parliament where it only holds a minority.